refactor(agents): remove interrogator, merge design Q&A into conductor

Confirmed via official Copilot CLI docs: there is no ask_user/elicitation
tool available to custom agents (only execute/read/edit/search/agent/web/
todo aliases exist). Subagent calls made through the 'agent' tool are
stateless — they run to completion and return one final result, with no
mechanism to pause mid-task for a live human reply.

This means conductor invoking interrogator via the agent tool could never
work: interrogator would run as a subagent regardless of whether conductor
itself was foreground or backgrounded, and subagents can't get real user
answers. That's why it was silently writing fabricated decisions.md/
questionnaire.md content instead of actually asking anything.

Fix: delete the interrogator agent entirely and fold its full Q&A
procedure directly into conductor's own DESIGN PHASE, run inline in
conductor's own foreground turn — never delegated. Updated the design
skill/prompt, AGENTS.md, README, QUICKSTART, and role-tier config to
match. Single entry point, no subagent path for anything that needs a
live human answer.

.github/prompts/design.prompt.md

@@ -1,10 +1,10 @@
 ---
-description: Run the lean harness design gate through the Interrogator agent or design skill.
+description: Run the lean harness design gate through the design skill or Conductor's DESIGN PHASE.
 ---
 
 # Design Prompt
 
-Invoke the `design` skill or `Interrogator` agent.
+Invoke the `design` skill, or invoke `Conductor` and run only its DESIGN PHASE section.
 
 Use `.agents/specs/<slug>/questionnaire.md` and `.agents/specs/<slug>/decisions.md`.
 If VS Code cannot run parallel subagents, proceed sequentially.